Unexpected connection: Rotation reversal tied to energy confinement saturation

Nov 11, 2011
Plasma fluctuations, that accompany the low density, low confinement regime and disappear promptly at the rotation reversal may provide the link between these two seemingly disparate phenomena. Credit: John Rice, MIT

Research on the Alcator C-Mod experiment at MIT has made an unexpected connection between two seemingly unrelated but important phenomena observed in tokamak plasmas: spontaneous plasma rotation and the global energy confinement of the plasma.

Self-generated flows, the spontaneous rotation which arises even when there is no external momentum input, can have a strong beneficial effect on plasma transport and stability. But in a , unlike most current experiments, there will be little or no external rotation drive—thus it is crucial to understand and be able to predict plasma rotation under these conditions. In the discharges studied at Alcator C-Mod, the flows can reverse direction at a very precise transition point, depending on plasma density and current.

This flow reversal turns out to be tightly connected to the confinement of the plasma. Since the early days of tokamak research (1970s), it was known that the energy confinement time (the ratio of the energy content of the plasma and the total input power) increased as the density of the plasma increased. This generated a great deal of excitement since one of the goals of fusion research was to operate at high density with good confinement, so this improvement was a bonus. At high enough density, however, the energy confinement stopped increasing.

The results of these experiments suggest that energy confinement and rotation reversal are closely related. At low density, where the energy confinement time increases with the density, the plasma rotates in one direction at roughly +5 km/s. Then at the critical density, the direction reverses direction to values around -20 km/s and the energy confinement saturates. The critical density depends on plasma conditions, increasing with plasma current and decreasing with machine size.

These observations reveal the fundamental connection between the two phenomena and how they both depend on the nature of the underlying plasma turbulence. One hypothesis is that at low density, the turbulence is driven by trapped electron modes, which strongly degrade the confinement and which propagate in a particular direction. As the density is raised, these modes are suppressed, and turbulence driven by ion temperature gradients dominates. These modes at higher regulate the confinement and propagate in the opposite direction. Evidence for this explanation is emerging from careful measurements of plasma fluctuations.

Explore further: Ultrafast imaging of complex systems in 3D at near atomic resolution nears

Provided by American Physical Society

4.8 /5 (5 votes)
add to favorites email to friend print save as pdf

Related Stories

I-mode powers up on alcator C-mod tokamak

Nov 10, 2011

A key challenge in producing fusion energy is confining the plasma long enough for the ionized hydrogen to fuse and produce net power. Suppressing plasma turbulence is one approach to this, but the resulting ...

A new spin on understanding plasma confinement

Nov 10, 2011

To achieve nuclear fusion for practical energy production, scientists often use magnetic fields to confine plasma. This creates a magnetic (or more precisely "magneto-hydrodynamic") fluid in which plasma is tied to magnetic ...

Upping the power triggers an ordered helical plasma

Nov 02, 2009

If you keep twisting a straight elastic string, at some moment it starts kinking in a wild way. Something similar occurs when one increases the electrical current flowing in a magnetized plasma doughnut: it ...

Tokamak experiments come clean about impurity transport

Nov 10, 2011

A fusion reactor operates best when the hot plasma inside it consists only of fusion fuel (hydrogen's heavy isotopes, deuterium and tritium), much as a car runs best with a clean engine. But fusion fuel reactions ...

Recommended for you

What's next for the Large Hadron Collider?

23 hours ago

The world's most powerful particle collider is waking up from a well-earned rest. After roughly two years of heavy maintenance, scientists have nearly doubled the power of the Large Hadron Collider (LHC) ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

Callippo
3 / 5 (2) Nov 14, 2011
I see, just another article about tokamak research. Suddenly, when it turns out, the cold fusion is feasible, all tokamak researchers are starting to publish frantically...;-) Is it organized propaganda - or just omnipresent fear of competition?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.